Do you mean I would like you to tell me the truth? If so, your attempt is almost perfect, except that it should be să-mi. Spuneți is used if you are talking to a group or a person you call dumneavoastră. If you're talking to someone you call tu, then it is să-mi spui.
And I think it would be more common to say Mi-aș dori instead of Aș dori here. Alternatively, aș vrea.
